(1) Field of the Invention
The present invention relates to a converting circuit and a controller for controlling the same, and more particularly to a converting circuit for reducing audible noise and a controller for controlling the same.
(2) Description of the Prior Art
FIG. 1 is a schematic circuit diagram of a DC-DC buck converter in the conventional arts, which is utilized for converting a DC input voltage Vin into a DC output voltage Vout. The DC-DC buck converter has a first switch Q1, a second switch Q2, an inductor L, a capacitor C, a voltage detector 50, a current detector RCS, and a controller, and the controller has a current sensing amplifier 10, an error amplifier 20, a pulse width modulated (PWM) comparator 30, and a skip mode controller 40. The first switch Q1 and the second switch Q2 are connected in serial between a DC input Vin and ground. The inductor L and the capacitor C, which are connected in serial between ground and a connecting point of the first switch Q1 and the second switch Q2, are utilized for generating the DC output voltage Vout. The current detector RCS is coupled to the inductor L for detecting the current on the inductor L and generates a current detecting signal CS. The voltage detector 50 is coupled to the capacitor C to detect the DC output voltage Vout and generates a voltage detecting signal VS.
A non-inverting terminal of the error amplifier 20 receives the voltage detecting signal VS and an inverting terminal thereof receives a reference voltage signal VREF. The output signal of the error amplifier 20 is compensated by a compensating circuit RCcomp at an output terminal of the error amplifier 20 to generate an error amplified signal EA. The PWM comparator 30 compares the error amplified signal EA with the sum of the current detecting signal CS and a slope compensating signal Slope and accordingly generates a comparing signal Comp. The skip mode controller 40 receives the comparing signal Comp and the current detecting signal CS and accordingly generates a first controlling signal UGATE and a second controlling signal LGATE for controlling the switching of the first switch Q1 and the second switch Q2, respectively.
When the DC output voltage Vout is lower than a predetermined voltage, the skip mode controller 40 turns the first switch Q1 on and turns the second switch Q2 off so that the current form the DC input voltage Vin charges the capacitor C for increasing the voltage level of the DC output voltage Vout. When the sum of the current detecting signal CS and the slope compensating signal Slope increases to reach the level of the error amplified EA, the first switch Q1 is turned off and the second switch Q2 is turned on so that the current from the inductor L continues to flow through the second switch Q2. When the current on the inductor is going to reverse (i.e.: the capacitor C is going to be turned from charging status into discharging status), the second switch Q2 is turned off but the first switch Q1 remains to be turned off. At this time, the DC-DC buck converter continues to provide electronic power to a load through the capacitor C. In the next period, when the DC output voltage Vout is lower than the predetermined voltage, the skip mode controller 40 turns the first switch Q1 on and turns the second switch Q2 off once again to charge the capacitor C for increasing the voltage level of the DC output voltage Vout. Thus, the cycle mentioned above repeats to allow the DC output voltage Vout stabled around the predetermined voltage.
If the load is so light that the DC output voltage Vout is still larger than the predetermined voltage in the above mentioned next period, the skip mode controller 40 will enter the skip mode from the normal mode as described above to keep the first switch Q1 and the second switch Q2 being turned off until the DC output voltage Vout declined below the predetermined voltage. However, if the time interval between the adjacent time points that the first switch Q1 being turned on is within the human hearing range, which is ranged between 20 Hz to 20 kHz, audible noise would be generated.